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Version:

August 20, 2016:
Revised: v1.0

Final Touches for my 2.5GHz 8-digit Frequency Counter

I've
been busy adding the finishing touches to my new Chinese-made frequency
counter. It uses one of those cheap modules purchased over the
Internet. To complete the counter, I designed and printed a
compact enclosure, and then set about the final assembly. A new
translation of the operating manual completed the job.

Introduction

I have
designed and built a number of different frequency counters over the
past twenty years. However, I don't have any frequency counters which
can count frequencies above 500MHz. That became apparent recently
when, several times, I’ve needed to measure some UHF
frequencies. I was forced to build a small prescaler module as a
temporary solution. That wasn’t an ideal solution for the longer term,
so I began exploring options to fix this.

Available Options

After
weighing up several published designs and a number of designs from
various magazines and websites, I took a closer look at the low cost
frequency counter modules now widely available from the usual Chinese
sources on the Internet. These fall into three main types; Simple 5
digit LED frequency counter modules operating up to about 50MHz, 8
digit LED counter modules operating up to 2.5GHz, and complete 8-digit
frequency counters with one or two inputs complete in a case with
integrated AC power supplies.

The 8-digit frequency counter
modules seemed to fit my requirements quite well, and the prices are so
low, it is nearly impossible to see how I could make one myself for the
same cost. While there are many suppliers, they all seem to offer the
same product, albeit with a choice of red, green or blue LED displays.
I decided to buy the red LED version, and within a few weeks, it was in
my hands.

While
the frequency counter module met all my technical and price
requirements, and I even had a suitable 12V 300mA power supply for it,
I needed some kind of enclosure for the counter module. I couldn't
leave the BNC input connector and the DC power connector just floating
around at the end of the somewhat thin connecting cables.

Designing and Building an Enclosure

There
are several ways to mount the counter module and connectors. The best
method would be to make a metal box since this would provide shielding
for the counter. I don’t have a suitable workshop or tools for that, so
I explored the alternatives. The most obvious solution is to buy a
simple metal box and mount the module and connectors in it. That’s
certainly the easiet approach, but all of the boxes that I could find
seemed to be much too large. They would take up far too much bench
space. Besides, they looked ugly!

I decided that the best
approach was to design a suitable 3D printed box. While the counter
module will not be shielded in this box, I did not feel that it would
be a significant problem in use. Also, I could custom design a much
more compact solution, hopefully one that was more attractive than
one of those simple square metal boxes.

My design was
completed as usual with DesignSpark Mechanical. This is a reasonably
easy to use free 3D design package which I really enjoy using. I
experimented initially with some folded paper shapes before coming up
with the final basic hexagonal design. This shape kept the overall
enclosure size to an absolute minimum while also allowing the counter
to sit on the workbench, either on its base or its back, as well as in
its usual place on top of some other test equipment.

Since
my printer is limited to 150mm in each axis, that set the maximum
possible enclosure size. Once the basic shape was determined, it became
clear that the best way to print it was in a vertical “tube”, with
separately printed end caps and buttons. The BNC connector is fitted at
the left hand side of the front panel.

Figure 3 : The end caps fit on each end of the case resulting in a compact enclosure

The counter module’s pushbuttons are quite small, and the new case required the switches to be each fitted with a short extension. These were just pushed into place on top of the existing pushbuttons.

Figure
4 : Two small printed caps are inserted through the front panel onto
the existing PCB switches once the module is mounted in the case

Standard
PLA filament was used to make the enclosure, printed in 0.2mm layers.
The industry-standard STL format files for the main case, the end caps
and the two button caps are all available for download (You will find
them at the end of this webpage, below).

Final Assembly

Once
all of the parts for the case were printed, the counter module was slid
into one end and mounted into place in the main case using four 6mm
long 3mm diameter cap-head screws. These must be purchased separately
because these are not supplied with the module.

Figure 5 : Inside the 3D-printed cae - There's not a lot of spare space, but everything fits without much difficulty.Make careful note of the colours of the wiring connections for theRF socket. Not ideal, but these are the connector cables supplied with this module.

The
module is supplied complete with two short wire leads, each with a PCB
connector to match the connectors on the module. The DC cable was
shortened to suit, soldered onto the DC socket, and pushed into place.
Although the DC connector is a tight fit in the case, a drop of
hot-glue was added to firmly hold the DC connector in place.

The
front panel artwork was printed on a colour laser printer, covered with
clear contact plastic film, and glued into place. Once this has been
fitted, the BNC connector can then be mounted. The short connecting
wires on one of the provided cables should be cut to a suitable length,
and then soldered in place on the fitted connector. This has to be done
carefully because too much heat from the soldering iron can melt the
case!

Figure 6 : I added provision for a standard DC socket into the enclosure design. It fits quite tightlyinto this case opening. A drop of hot-glue inside ensures that it will stay permanently in place.

The
switch caps were then inserting through the holes in the front of the
counter box and pressed into place. Finally, the end caps were pushed
on. A drop of glue can be added if necessary to hold these firmly in
place.

Counter Labels

The
various front and back labels were designed and printed on paper using
a colour laser printer. The various holes were then cut out with a
sharp hobby knife, and each label then covered with self-adhesive
transparent plastic. With the exception of the main display opening,
the clear plastic film covering all of the other holes was trimmed off
and the labels then glued to the enclosure.

Figure 7 : Panel artwork is available (below, in the Download section) in different colours.

Operation

The
PDF manual from the suppliers left a great deal to the imagination, in
part because it was all in Chinese. Not being able to find a good
English version online, I translated the Chinese text into English, and
redid the flow diagrams for good measure. A copy of this manual
complete with the new flowcharts can also be downloaded below.

There
are two buttons on the right edge of the counter. The upper button is
the “SET” key. This allows programming of an offset frequency, for
example when the counter is used as a frequency display in a receiver
or transceiver, and the selection of the counter input channel. A
digital filtering mode, LED display brightness and least significant
digit blanking functions can also be selected with this button.
The lower “” button is used to make subsequent selections. For most
situations, these two buttons can be ignored.

Final Comments

While
there are two input channels in the counter module, one using a
discrete component RF amplifier for frequencies up to about 50MHz and
the other channel having an IC prescaler for frequencies up to 2.5GHz,
the inputs for these channels are simply combined onto the counter’s
single PCB RF connector. This is a little "agricultural", but it does
avoid the need for additional circuits and parts to switch between the
inputs. It comes at the cost of reduced sensitivity and a somewhat
uncertain input impedance.

I briefly looked at separating
the input channels to allow the use of two input connectors. This would
improve the input matching and sensitivity. However, the location of
one of the channels under the permanently soldered LED displays made
that impractical, so the module was left unmodified. It seems to work
well enough for my purposes, so that was probably a very good choice.

I
was able to adjust the counter’s calibration preset (See the manual
below for more details) to match my GPS-based frequency reference,
making it accurate within 1Hz at 20MHz. That was done after I first
turned the counter on and I let it stabilize to room temperature for
about 10 minutes.

In practice, the counter has proven to
be sensitive, accurate, and easy to use. I have no complaints,
especially given the excellent price. In particular, I like the
convenient size and shape of my new 3D printed counter enclosure.

I hope you give it a try too.

Downloads:

Counter Box 3D Files: This ZIP file includes all of the industry-standard STL files for the main case, the end caps and the pushbutton extensions

ENGLISH version of the operating manual: This
PDF file is my translation of the original Chinese manual and includes
translated diagrams and some additional information about
sensitivity test resultsPanel Labels This
JPG file contains the artwork for all of the labels used on the
counter. There are everal different colours available in this file